1. Field of the Invention
This invention relates to apparatus and methods for monitoring colour changes in what will be referred to herein as a target, i.e. anything in or on which a change of colour may take place. The term "colour" is to be understood to mean the wavelength of light, emitted or reflected according to the context, whether or not it happens to be in the visible part of the spectrum.
2. Discussion of Prior Art
The problem of detecting over a substantial period of time the changes in chemical composition occurring in a gas or liquid flow, or indeed the detection of the change in any physical quantity which can be made to manifest itself as a colour change, is advantageously addressed by apparatus for monitoring colour changes. Such apparatus requires a high level of stability and fault tolerance if it is to be in operation over a long period of time.
One known system that can be applied to the measurement of absolute colour intensities over a period of time is an optical device for measuring physical quantities and is disclosed in UK Patent Application No. 2,025,608 (corresponding to U.S. Pat. No. 4,281,245). This device comprises generally two sources of different wavelengths of light which are optically connected to an optical transducer. The transducer comprises an optical filter having spectrally varying absorbing, transmitting or reflecting properties which are influenced by the physical property in question. The transducer is assembled in such a way as to ensure that one of the wavelengths of light (the reference signal) is substantially less dependant on the measured quantity than the other (the measuring signal). The corresponding two output signals of the transducer are then divided and in this way a stablised measurement is obtained.
This known device is capable of measuring a sequential series of absolute values of the physical quantity over a period of time. Errors common to the paths of both wavelengths of light are removed by the divider, however other errors which vary with time will be incorporated into the measurements obtained obscuring genuine changes in the value of the physical quantity. Such errors can be caused by drifts in the sensitivity of the sensors and in the intensity of the light source due to ageing and other inconsistencies which are likely to occur over a substantial period of time. One way of overcoming this is by repeated calibration procedures which would necessarily halt the continuous monitoring of the target and is therefore inconvenient.